In recent years, electronic apparatuses, particularly wireless and cable telecommunications apparatuses, generally include circuits, such as a digital circuit and a switched capacitor circuit, each of which operates in synchronization with a clock signal. In such circuits, an instantaneous current flows in synchronization with a clock signal. This causes noises each having a high level and each having a frequency of the clock signal (a fundamental wave) or a frequency of one of higher harmonics. The noises are mixed into a signal, thereby causing a deterioration in signal-to-noise (S/N) ratio of the signal. This phenomenon often leads to a problem, particularly in communications receivers.
FIG. 8 is a block diagram illustrating an example of a conventional electronic device 100 including a direct conversion receiver 110 and a clock synchronization circuit 106. A RF signal received via an antenna 101 is amplified by an amplifier 102. Next, a mixer 103 carries out a frequency conversion by mixing the radio frequency (RF) signal thus amplified and a local oscillator signal having a frequency fLO so that the RF signal thus subjected to the frequency conversion is converted into a baseband signal. The baseband signal supplied from the mixer 103 is attenuated by a low-pass filter 104 so that its frequency components, other than a frequency component falling within a target signal band, are attenuated. The baseband signal thus attenuated is amplified by an amplifier 105, and is then outputted via an output terminal OUT.
The output signal of the amplifier 105 contains a direct-current (DC) offset. The DC offset is fed back by a DC offset canceller 107 to the output of the mixer, and is thus removed. The clock synchronization circuit 106 is used either to carry out part of a function of the direct conversion receiver or to carry out another function different from the function of the direct conversion receiver. In either case, the operations are carried out in synchronization with a clock signal having a clock frequency fclk. This causes the higher harmonic waves, whose respective frequencies are respective integral multiples of the clock frequency fclk, to be undesirably mixed into a path for the RF signal received via the antenna 101. The higher harmonic waves are mixed into the path via, e.g., power supply wiring, a common substrate, coupling of electric fields or magnetic fields, or an electromagnetic wave.
FIG. 9 shows an, example of a signal spectrum at a RF signal input point A in the receiver of FIG. 8. FIG. 9 specifically shows: a target signal which has been subjected to a modulation and occupies a predetermined frequency band; the clock signal, having the clock frequency fclk, which is undesirably mixed into at the RF signal input point A; and higher harmonic waves of the clock signal having the clock frequency fclk.
These signals and the local oscillator signal having the frequency fLO are mixed by the mixer 103 of FIG. 8, so as to be converted into a baseband signal having a signal spectrum shown in FIG. 10.
It is assumed in FIG. 10 that (N−1)×fclk<fLO<N×fclk is satisfied. FIG. 10 shows that an in-band spurious component is generated at a frequency N×fclk−fLO by mixing an N-th higher harmonic wave having a frequency N×fclk and the local oscillator signal having the frequency fLO.
FIG. 11 is a block diagram illustrating an example of a conventional electronic device 120 including a heterodyne receiver 130 and a clock synchronization circuit 126. A signal spectrum of (i) a target signal at a RF signal input point A in the receiver 130 of FIG. 11 and (ii) higher harmonic waves of a clock signal having a clock frequency fclk, which are mixed into at the RF signal input point A, is identical to that shown in FIG. 9.
According to the electronic device 130, a RF signal received via an antenna 121 is amplified by an amplifier 122. Next, a mixer 123 carries out a frequency conversion by mixing the RF signal thus amplified and a local oscillator signal having a frequency fLO so as to be converted into an intermediate frequency signal. After that, the intermediate frequency signal supplied from the mixer 123 is attenuated by a band-pass filter 124 so that its frequency components, other than a component falling within a target signal band, are attenuated. The intermediate frequency signal thus attenuated is amplified by an amplifier 125, and is thus outputted via an output terminal OUT.
The clock synchronization circuit 126 is used either to carry out part of a function of the superheterodyne receiver 130 or to carry out another function different from the function of the superheterodyne receiver 130. In either case, the operations are carried out in synchronization with a clock signal having a clock frequency fclk. This causes the higher harmonic waves, whose respective frequencies are integral multiples of the clock frequency fclk, to be undesirably mixed into a path for the RF signal received via the antenna 121. The higher harmonic waves are mixed into the path via, e.g., power supply wiring, a common substrate, coupling of electric fields or magnetic fields, or an electromagnetic wave.
These signals and the local oscillator signal having the frequency fLO are mixed by the mixer 123 of FIG. 11, so as to be converted into a baseband signal having a signal spectrum shown in FIG. 12.
It is assumed in FIG. 12 that (N−2)×fclk<fLO<(N−1)×fclk is satisfied. FIG. 12 shows that an in-band spurious component is generated at a frequency N×fclk−fLO by mixing an N-th higher harmonic wave having a frequency N×fclk and the local oscillator signal having the frequency fLO.
In order to address the problems, a method is known in which spurious components are removed by providing a notch filter having notch frequencies identical to respective frequencies of higher harmonic wave spurious components.
Patent Literature 1 discloses a method for removing spurious components in an orthogonal frequency division multiplexing (OFDM) receiver. According to the description as to the method, (i) a received signal, which has been subjected to a frequency conversion by a frequency converter circuit, is converted by a fast Fourier transform (FFT) circuit into a frequency-domain signal so that frequencies of respective spurious components which are mixed into the received signal are detected, (ii) notch frequencies of a variable notch filter are controlled so as to match the respective frequencies of the spurious components, thereby allowing a removal of the interfering spurious components.
Patent Literature 2 discloses as follows. An apparatus including a microcomputer causes unnecessary signals such as components of higher harmonic waves of a clock signal which are generated due to a mechanism in which the apparatus operates in response to the clock signal supplied from the microcomputer. Such unnecessary signals interfere with a signal which the apparatus should proceed. In view of the circumstances, a frequency of the clock signal is temporally varied so that energy of the higher harmonic waves is dispersed, thereby reducing a noise level.
Citation List
Patent Literature 1
Japanese Patent Application Publication, Tokukai, No. 2006-174218 A (Publication Date: Jun. 29, 2006)
Patent Literature 2
Japanese Patent Application Publication, Tokukaihei, No. 11-143572 A (Publication Date: May 28, 1999)